A gold-to-gold interconnection system eliminates the intermetallic problems commonly associated with the use of two different metals which are ultrasonically bonded, such as differential diffusion creating Kirkendall voids or "purple plague", which pose reliability problems when two types of metals are ultrasonically bonded. Gold wire bonding and tape automated bonding (TAB) are fairly common and reliable methods of interconnection on integrated circuits and between integrated circuits, but usually require heating of the integrated circuit or substrate, commonly at temperatures of 200-250.degree. C. Various methods of making electrical connections using TAB tape lead frames to connect integrated electrical components, tin/lead-to-tin/lead solder reflow methods (both single point and gang thermode), or tin-to-gold eutectic thermal compression methods (single point and gang thermode) are all methods well known to those skilled in the art.
Gold-to-gold ultrasonic diffusion bonding of electrical interconnects between TAB tape lead frames and printed circuit board substrates is, however, a difficult interconnect process to achieve because it requires tighter control of the bonding variables and other factors influencing the strength of bonds. As a result, such bonds are not suited for a reliable high volume production connection process using known bonding methods.
Gold-to-gold thermal compression bonding has typically been used for TAB bonding of lead frames to substrates by either single point or gang bonding modes of operation, but it is severely limited by the slow cycle times and extremely high temperatures (up to 500.degree. C.) required to produce reliable bonds. As a result, thermal compression bonding is not desirable, or even possible, in high volume production electrical interconnect applications of integrated circuits with gold plated TAB lead frames connected to gold plated printed circuit board substrates of epoxy resin material construction which require high through-put and low temperature levels to avoid damaging the connected components.
Gold-to-gold ultrasonic diffusion bonding of electrical connections between TAB tape lead frames and printed circuit boards is difficult to achieve without damaging components located on the boards when deep access bonding is required. Problems include bonding tools, typically available only in industry standard lengths of 0.375 and 0.437 inches when being utilized for TAB bonding applications, which do not provide enough vertical clearance between the components and the ultrasonic transducer. Increasing the length of a standard bonding tool and using a standard transducer reduces excursion control of the bonding tool tip, resulting in random, nonrepeatable motion of the tip which negatively impacts bond strength and repeatability. In addition, another serious problem is transducer wear at the bonding tool/transducer tip interface which is attributable to the longer tool length needed to perform deep access bonding. The standard transducer design includes a circular bore to hold the bonding tool which rapidly wears into an elliptically-shaped hole when the required ultrasonic power is supplied of the transducer. That distortion to the transducer only adds to the loss of excursion control. Deep access gold-to-gold ultrasonic diffusion bonding, however, requires an even tighter level of control of the ultrasonic bonding variables and factors influencing the bondability--control the current art does not provide.
An even more difficult process is deep access bonding to bonding pads located on epoxy resin substrates which cannot be heated without damaging the components on or in the board. As a result, no reliable gold-to-gold TAB interconnect process involving non-bumped gold pads on epoxy resin substrates or PTFE construction printed circuit board substrates (which cannot tolerate elevated temperatures) is known in the art. In addition, deep access bonding between integrated circuit bodies located on such printed circuit boards has also been unknown in standard practice.
Other problems encountered in gold-to-gold ultrasonic bonding with an unheated, i.e., room temperature, substrate is control of the high power required to perform such ultrasonic bonding applications. The high power, like the increased bonding tool length required for deep access bonding, also reduces excursion control of the bonding tool tip, resulting in random, non-repeatable motion of the tip which negatively impacts bond strength and repeatability. The higher power also dramatically increases transducer wear at the bonding tool/transducer tip interface beyond that seen when only a longer bonding tool is used. The resulting gold-to-gold deep access bonding on an unheated substrate cannot be accomplished at rates and reliability levels required for high volume production.